There is a pretty constant debate about whether or not Aspartame will have any negative side effects. As karl says, moderation is probably the best bet. Chanel, I think you are going to be hard pressed to find actual and credible scientific studies to back up any of the claims you have made about the possible things Aspartame has been "shown to cause". There are a lot of myths and lot of loose talk on this subject being tossed around without any backing.
This is not to say that I suggest Aspartame is some miracle sweetener or anything like that, but both FDA and the CDC have extensively looked into these types of claims made about the side effects of Aspartame and have not found any facts that indicate any serious health risks. It's not a great idea to tout panic-driven suspicions as fact - or to bother believing it as such - unless you have some type of cedible scientific study to back this data up.
That being said, Chanel and aevans are both 100% correct, nothing can replace water and wherever possible this should be your drink of choice before anything with artificial sweeteners (or anything else that completely lacks any nutritional value). Juice is also good (again, like everything else in life, in moderation). The jury is still out and studies are still being conducted to determine any long term risks from artificial sweeteners, so do yourself a favor and error on the side of caution and use these products in moderation, but feel free to not flee screaming at the sight of them also.